1. Field of the Invention
The present invention relates to a micro-electromechanical system (MEMS) structure. More particularly, the present invention relates to a MEMS transducer formed on a flexible substrate, a manufacturing method thereof and a flexible MEMS wireless microphone incorporating the MEMS transducer.
2. Description of the Related Art
As the need for very small devices increases, semiconductor processing technology using micromachining techniques is employed to integrate micro devices. The field of micro-electromechanical systems (MEMS) is a field of manufacturing and testing miniature sensors and actuators, which have sizes on the order of micrometers (μm), and electromechanical structures using micromachining technology applied in semiconductor processing, particularly, in integrated circuit technology.
The micromachining technology employed in the MEMS is largely divided into two categories. The first micromachining category is bulk micromachining by silicon bulk etching. The second micromachining category is surface micromachining by depositing a film of polycrystalline silicon, silicon nitride and silicon oxide on silicon and etching the deposited film according to a predetermined pattern to form a structure. For example, the formation of an ultra small microphone manufactured using a MEMS process is achieved using a diaphragm transducer formed by the bulk micromachining technology.
FIG. 1 illustrates a cross-sectional view of a conventional MEMS transducer. As shown in FIG. 1, the conventional MEMS transducer includes a diaphragm layer of silicon nitride, a SiO2 layer coated by a chemical vapor deposition (CVD) process, a piezo film of zinc oxide (ZnO) and an upper electrode and a lower electrode on a silicon wafer (Si). The CVD process to form a silicon nitride thin film and silicon oxide layer on the silicon wafer is a high temperature process requiring a process temperature of about 780° C. to 850° C. Therefore, it is impossible to use a flexible polymeric material other than the silicon wafer as a material for the substrate.
Meanwhile, as the information communication industry develops, demand for a hand-held or wearable information terminal is similarly increasing. This increase in demand is due in part to the applications of such information terminals being implemented into diverse fields, such as medical, service, entertainment, military, and information communication. For convenience in using these information terminals, the components of these terminals should have excellent characteristics in terms of mobility and wearability. In particularly, in order to realize a wearable system, a flexible system structure is essential. Therefore, a technology to integrate a functional structure and other electric parts together on a flexible substrate is needed.
As a flexible substrate, metallic thin films or polymeric materials are used. Polymeric materials are more suitable for use in an electronic system. Polymeric materials, however, have a low melting point in the range of 500° C. or less. Thus, when polymeric materials are subjected to a process for forming a thin film at a high temperature, the polymeric materials deteriorate. Therefore, polymeric materials are not suitable for use as a material for the substrate, such as a wafer, in a process for manufacturing MEMS, which requires a process temperature that is higher than the melting point of the polymeric materials. In practice, silicon MEMS and semiconductors, which are widely used and have excellent characteristics in terms of performance and degree of integration, are generally produced by methods including a high temperature process of at least 500° C. Therefore, the substrate of a high molecular (polymeric) material, which is needed for a flexible system structure, cannot be used.
Specifically, a conventional MEMS structure is formed by depositing a thin film on a substrate by chemical vapor deposition (CVD), followed by an etching process. However, since a very high temperature is needed to form a high-utility thin film by CVD, a low-melting point substrate, such as a polymer, glass, and the like, cannot be used.
In order to overcome such problems, as shown in FIG. 2, a conventional method produces a flexible device by forming a sensor device 30 on a silicon substrate 10 using a silicon MEMS process, cutting between silicon islands from a backside of the silicon substrate 10 and then depositing a polymer 11. However, this method has disadvantages in that the conventional silicon MEMS process adopting a high temperature process is used and a polymer process is additionally performed in a final step, thereby increasing the complexity and the cost of the entire manufacturing process.